Many silicon single crystals as substrate materials for semiconductor devices are manufactured by the CZ method. In the CZ method, a raw material such as polycrystalline silicon is charged into a quartz crucible and heated in a chamber for melting. Then, a seed crystal attached to the lower end of a pull-up shaft is lowered from above the quartz crucible and is dipped into the silicon melt. Then, the seed crystal is gradually lifted while rotating the seed crystal and the crucible, whereby a large single crystal grows at the lower end of the seed crystal.
A multi-pulling method is also known as a manufacturing method for a silicon single crystal according to the CZ method (see, e.g., Patent Document 1). In the multi-pulling method, after the pulling of the silicon single crystal is completed, additional silicon raw material is supplied to the same quartz crucible and is melted, and a subsequent silicon single crystal is pulled up from the obtained silicon melt. Such a raw material supplying step and a single crystal pull-up step are repeated to manufacture a plurality of single silicon crystals in one quartz crucible. According to the multi-pulling method, cost of the quartz crucible per silicon single crystal can be reduced. Further, the frequency of disassembling the chamber and exchanging the quartz crucible can be reduced, thus allowing improvement of operation efficiency.
There are known various methods of heating the silicon raw material. For example, in the method described in Patent Document 2, after a raw material is melted by upper and lower heaters disposed outside a crucible, power supply to the lower heater is immediately reduced to zero to form a solid layer of the raw material, and a silicon single crystal is grown from a melt layer coexisting on the solid layer. Further, in the method disclosed in Patent Document 3, a division heater constituted of a side heater for heating the periphery of a crucible and a bottom heater for heating the bottom portion of the crucible is used to melt a raw material in the crucible in a short time. Further, in the method disclosed in Patent Document 4, in order to suppress power consumption for melting a silicon material, a heater output is suppressed to a low level until the temperature of the silicon raw material exceeds a reference temperature ranging from 200° C. to 300° C., and the heater output is increased after the temperature of the silicon raw material exceeds the reference temperature.
A reduction in carbon concentration in a silicon single crystal is one of important issues. Because, it is known that carbon in the silicon single crystal accelerates oxygen precipitation, and an oxygen precipitate has influence on device performance (e.g., increases current leakage). For example, it is reported that when carrier lifetime is controlled by electron beam irradiation and annealing in an IGBT (Insulated Gate Bipolar Transistor), carbon may affect device characteristics such as saturation voltage.
An increase in carbon concentration in the silicon single crystal is considered to be brought about by CO gas mainly generated from a carbon heater. The CO gas is generated when SiO gas evaporated from a silicon melt reacts with high-temperature heater, and the generated CO gas adheres to an unmelted silicon raw material. Then, carbon is dissolved in the silicon raw material being melted, with the result that the carbon concentration in the melt is increased, which in turn increases the carbon concentration in a silicon single crystal pulled up from the silicon melt. Particularly, in the above-described multi-pulling method, the problem of carbon contamination becomes prominent since the carbon concentration in the single crystal increases as the number of times of pulling up the single crystal increases.
In order to pull up a silicon single crystal having a low carbon concentration, Patent Document 5 proposes a method in which a rectification member for increasing the flow rate of inert gas is provided at the upper portion of a carbon susceptor that retains a quartz crucible. The increase in the flow rate of the inert gas by the rectification member allows CO gas generated from a heater to be discharged efficiently.